Punching using a die and a punch is conventionally known as a method of forming holes with high quality and at high productivity. For example, Japanese Unexamined Patent Application Publication No. 2002-102977 (U.S. Pat. No. 6,678,955) describes an apparatus and a method of forming an injection hole of a fuel injection nozzle by punching. Electrical discharge working and laser beam working are also generally known as methods of forming holes.
In general, a punch receives a large processing load when being pressed into a work piece for punching. If the work piece is elastically deformed and expanded by the punch when the punch is pressed in the work piece, the work piece sticks to the punch due to a reaction force when the punch is removed from the work piece. As a result, the punch receives a tensile stress.
To form a hole in which a ratio of depth L to an inner diameter D is equal to or greater than two (L/D≧2), a punch having a thin and long columnar shape is used. The thin and long punch will be easily bucked under a large processing load when being pressed into the work piece. Also, the thin and long punch will be easily ruptured due to a tensile stress when being removed from the work piece. Thus, it is difficult to form the hole having the relationship of L/D≧2 by punching.
On the other hand, the hole having the relationship of L/D≧2 can be formed by the electric discharge working or the laser working. However, the electric discharge working takes a long time to form the hole, resulting in deterioration of productivity and an increase in manufacturing costs.
In the laser working, the hole can be formed in a short time, as compared with the electric discharge working. However, dimensional accuracy is deteriorated and an inner surface of the hole is generally rough, as compared with the electric discharge working. Therefore, it is difficult to achieve sufficient processing quality.